1. Field of the Invention
This invention is concerned with a downhole acoustic televiewer. Disclosed is a method for discretely controlling the amplitude level of the excitation voltage applied to an acoustic transducer for the purpose of eliminating the interference between the caudal portions of an outgoing pulse and the returning echo signals reflected from the side wall.
2. Discussion of the Prior Art
When boreholes are drilled deep into the earth, it is of great interest geologically to examine the texture and composition of the material making up the borehole wall. One method would be to lower an optical television camera into the hole to a desired depth. Such a camera would be designed to circumferentially scan the side wall as the camera is drawn back up the hole to the surface. Small faults, dip of the strata, vugs, color and texture could be displayed on a monitor for study. Unfortunately, the drilling fluid used in drilling most boreholes, such as oil wells, is opaque to optical radiation. Therefore, down-hole formation-scanning tools use ultra-sonic radiation in place of optical radiation for imaging the borehole side-wall structure.
Typically, a down-hole acoustical scanner consists of one or more piezoelectric transducers mounted on a rotating head housed in a sonde. For purposes of this exemplary discussion, it will be assumed that the borehole is substantially vertical although horizontal holes are not excluded. The sonde may be ten feet or so long and three or four inches in diameter. The transducer is mounted in the sonde, behind an acoustic window, in a pressure-compensated, oil-filled cell. Other compartments in the logging sonde include electronics for actuating the transducer, for controlling the rotary head and for interfacing the electronics with control and display apparatus on the surface through a multiconductor logging cable.
In operation, the transducer(s) is caused to circumferentially scan the borehole sidewall at an exemplary scanning rate of six, 360.degree. scans per second as the sonde is passed through the borehole at, perhaps, five or ten feet per minute. A flux gate magnetometer in included in the sonde. The beginning of a scan occurs each time the rotary head is aligned with magnetic north. In some instruments, a three-axis accelerometer is used in place of, or in addition to the magnetometer. The transducer thus scans the borehole sidewall in a continuous spiral from the total depth to the surface.
The transducer is pulsed at a rate of 125 to 250 pulses per scan to provide an equal number of data samples per scan. The pulse frequency varies from 250 kHz to 2 mHz. Preferably the lower frequency is used to get better penetration through the borehole fluid which is highly attenuating at higher pulse frequencies. The driving energy may be coupled to the transducer(s) mounted on the rotating head through a rotary transformer. The vertical resolution between scans depends on the rate at which the sonde is passed through the borehole. Typically the vertical resolution is about 0.3 inch per scan.
The transducer element(s) preferably is focused. Because the borehole may include several different diameters, two transducers, having different focal lengths may be mounted on the rotary head. A short-focus element is used for hole diameters up to about six inches; the longer-focus unit is used in a hole with a greater diameter. Use of focused transducer elements minimizes beam spreading thereby providing improved horizontal resolution which is of the same order of magnitude as the vertical resolution. A third, fixed transducer is provided for use with a mud cell to measure the instantaneous drilling-fluid (mud) velocity.
A piezoelectric transducer may act as an acoustic transmitter or an acoustic receiver. At each sample time, the transducer, switched to the transmit mode, sends out a pulse as above described. The acoustic pulse propagates through the borehole fluid and is reflected from the borehole wall. Subsequently to pulse transmission, the transducer assumes a listening mode whereupon it receives the reflected pulse, reversed in phase with respect to the outgoing pulse, as a data sample. The received acoustic pulse is converted to an electrical signal which is delivered to data-processing circuitry on the surface. The transducer thereupon reverts back to the transmit mode for the next sample.
The quantities of interest are the time of flight and the relative amplitudes of the respective reflected echo pulses. The flight time, multiplied by the fluid velocity is a measure of the distance between the transducer and the sidewall, that is, the tool can serve as an acoustic caliper. The echo-signal amplitude may be interpreted as a function of the texture as well as the composition of the sidewall material as estimated from the characteristic acoustic impedance thereof. The respective data samples from a plurality of scans may be processed and displayed as a type of tomogram as a function of depth when cut along the north line and laid out flat. A description of a conventional system is found in a brochure "Circumferential Borehole Imaging Log", published by the assignee of this invention.
The pulser circuit typically employs a moderate-sized capacitor that is connected across a transformer through a normally-open switching device. The capacitor is charged. When the switch is closed, the capacitor discharges through the transformer thereby exciting the transducer to generate an acoustic pulse. Ideally, the excitation voltage would be a spike, that is, a Dirac function. But the capacitance/inductance parameters of the circuit cause the circuit to resonate. The excitation voltage, and hence the outgoing acoustic pulse, degenerates to an initial pulse followed by a caudal wave train of significant length in time.
The caudal wave train exhibits an exponential amplitude-decay rate. That phenomenon is sometimes referred to as ringdown. The ringdown time is defined as the time required for the wavetrain amplitude to decay to an arbitrarily-selected lower amplitude limit such as 40 to 60 dB down from maximum amplitude. The excitation voltage, v, is given by the well-known formulation: EQU v=Ve.sup.-at sin.omega.t, (1)
where
V=peak excitation voltage,
a=damping coefficient,
t=time,
.omega.=2.pi.f, and
e=natural logarithm base.
The output power of the transducer needs to be as high as possible; the power level is a function of the excitation voltage. In the above formulation, since the damping coefficient, a, is fixed by the circuit parameters, the ringdown time is also directly proportional to the peak excitation voltage V. Losses through the acoustic window in the sonde, losses and scattering through the highly attenuating borehole fluid and acoustic absorption by the sidewall material cause a 35 to 40 dB energy loss in the returned echo signal. Thus, if t.sub.e is the arrival time of the echo signal, the ringdown time must be optimized such that v&lt;&lt;v.sub.3, where v.sub.e is the signal level of the echo. If the inequality is not satisfied, trailing portions of the outgoing wavetrain will destructively interfere with the incoming echo signal.
Many prior-art acoustic scanning systems employed a transducer excitation frequency on the order of 2 mHz. Such systems usually employed some sort of AGC or a ramped gain function to amplify the weak reflected signal. See for example, U.S. Pat. No. 4,691,307 issued Sep. 1, 1987 to Rambow; U.S. Pat. No. 4,855,965, issued Aug. 8, 1989 to Rambow et al; and U.S. Pat. No. 4,984,221, issued Jan. 8, 1991 to Dennis. At frequencies in the megahertz range, the ringdown time is quite short and is not of concern.
The choice of excitation frequency is a compromise between the need for signal penetration through the drilling fluid using a longer-wavelength pulse and the need for spatial resolution that is achievable using shorter wavelengths albeit at the expense of higher signal transmission losses. In the implementation herein disclosed, an excitation frequency of 250 kHz is employed as a preferable compromise.
For a small borehole diameter such as 4.75 inches and in the presence of a drilling fluid that is characterized by a fast acoustic propagation velocity, the acoustic-pulse round-trip flight time is about 33 microseconds (.mu.s). At a transducer excitation frequency of 250 kHz, the period of one cycle of the caudal wavetrain is 4 .mu.s. An outgoing acoustic wavetrain having a ringdown time of 40 .mu.s (10 cycles) could destructively interfere with the weak incoming echo pulse. The transducer excitation voltage must be reduced to shorten the ringdown time. But as the diameter of the borehole increases, the transducer excitation voltage must be increased proportionately.
One method that was previously used to select the proper ringdown time required that the operator remove the sonde from the borehole and to empirically manipulate an iron slug with respect to an inductive circuit. But in the event that the diameter of the hole changed after the initial adjustment, it was not possible to re-tune the pulser circuit from the surface with the sonde still in place; the sonde first had to be withdrawn from the borehole, an expensive and time-consuming task.
There is a need for a system for discretely optimizing the ringdown time of the transmitted acoustic pulse to prevent interference with the returning echo signal while maintaining maximum signal output power commensurate with the borehole diameter. The system should be operator-controllable to accommodate abrupt changes in the borehole parameters.